The purpose of a catalytic converter is to facilitate the conversion of pollutant materials in internal combustion engine exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxides, ozone, etc. to carbon dioxide, water and other harmless gasses. Conventional catalytic converters utilize a ceramic honeycomb monolith having square, triangular, or circular openings straight-through openings or cells, catalyst coated alumina beads, or a corrugated thin metal foil honeycomb monolith, having a catalyst carried on or supported by the surface, which surface is, in the case of the thin metal honeycomb monolith, typically washcoated with one or more refractory metal oxides, e.g., alumina (gamma), ceria, lanthia, or combinations thereof, and a catalyst. The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such metals. The catalyst may also be manganese hexa-aluminate developed by Kobe Steel, Ltd. The catalyst catalyzes a chemical reaction whereby the pollutant material is converted to a harmless by-product which then passes through the exhaust system to the atmosphere.
However, this conversion is not efficient initially when the exhaust gases are relatively cold. To have high conversion efficiency at, for example, start-up, the catalyst and the surface of the converter with which the exhaust gases come in contact must be at a minimum elevated temperature, e.g., 390 F for carbon monoxide, 570 F. for volatile organic compounds (VOC), and 1000 F for methane or natural gas. Otherwise, the conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. Once the exhaust system has come to its normal operating temperature, the catalytic converter is optimally effective. Hence, it is necessary for the relatively cold exhaust gases to contact hot catalyst to effect satisfactory conversion at start-up. A thin metal honeycomb having a catalyst deposited on the surface thereof is especially adapted to this purpose in that it can be heated readily by electrical means as later described herein.
To achieve rapid heating of the catalyst in a metallic monolith by electrical means, it is necessary to draw a large amount of power from a voltage source or another source of electrical energy, e.g., a battery or a capacitor device, such as new Isuzu "electric power storage" device developed by Isuzu Motors Ltd., for a short period of time until the desired catalyst temperature is reached. In an automotive vehicle, for example, this source of energy is usually a 12 volt or 24 volt battery, although a battery system supplying up to as much as 108 volts may be used herein. To accomplish a high power draw on a storage battery system, it has been found that one or more actuatable solid state switches connected in parallel, such as metal oxide semiconductor field effect transistors (MOSFETs) together with means for actuating such devices in unison (a gate driver) may conveniently be used. Such a system enables drawing high power loads for a short period of time to achieve the desired catalyst temperature in from 2 to 30 seconds. Reference may be had to our copending application Ser. No. 587,219 filed Sep. 24, 1990 for details of a suitable power control system useful herein.
Reference may be had to U.S. Pat. No. 3,768,982 to Kitzner dated Oct. 30, 1973. In this patent heat from a centrally located electric heater is transferred by conduction through a monolithic catalyst support to heat the catalyst to optimum operating temperature. Reference may also be had to U.S. Pat. No. 3,770,982 to Kitzner dated Oct. 30, 1973 which discloses a central electrically heated core within a ceramic monolith, heat being transmitted by conduction to the catalyst contained in the openings of the ceramic monolith. The heating core is formed of metal sheets, one corrugated and the other flat, coated with alumina and also bearing a catalyst. The metallic core is heated electrically by virtue of its own electrical resistance. However, heating by conduction requires too long a period (a matter of minutes) to be practical in solving the problem of atmospheric pollution at start-up.
Reference may also be had to U.S. Pat. No. 4,711,009 to Cornelison, et al dated Dec. 8, 1987 for details of a process for the preparation of a continuous corrugated thin stainless steel strip having a wash coat of alumina (gamma) on at least one surface of the strip, and a noble metal catalyst deposited on the resulting surface thereof. This patent is incorporated herein by reference thereto.
Reference may also be had to International PCT publication numbers WO 89/10470 and WO 89/10471 each filed Nov. 2, 1989 which disclose electrically conductive honeycomb catalyst support units useful in automobiles. S-wound cores are disclosed in these publications. No brazing between corrugated thin metal foil layers is disclosed. To obtain a suitable resistance between 0.03 and 2 ohms, the honeycomb body is subdivided electrically, cross-sectionally and/or axially, by gaps and/or electrically insulating intermediate layers or coatings so that at least one path having the desired resistance is obtained. Heating is controlled by a timed relay. Separate catalytic converters in the exhaust line, one or more electrically heatable, the other conventional, are disclosed. The minimum resistance disclosed is 0.03 ohms which when placed in a 12 volt electrical system as disclosed, may be expected to draw no more than about 300 amps at the catalyst when the various voltage drops and resistances are considered. This power level (about 2700 watts) has been found to be too low for effective rapid heating of electrically heatable catalytic converters within from 2 to 30 seconds. Even if the monolith could be heated rapidly at this power level, the catalyst would rapidly cool to below the "light off" temperature after the engine is started because of the initial cool exhaust gas from the engine. To counteract this cooling effect, usually more than 2700 watts of power are required after engine start-up. It should be noted that on some vehicles, this cooling to lower temperature is not important if it occurs during a period of time when the engine exhaust emissions are low, in which case a lower power level may suffice. Moreover, subdivision of the monolith into a plurality of discs or units for connection in series is not necessary.
In the following description, reference will be made to "ferritic" stainless steel. A suitable formulation for this alloy is described in U.S. Pat. No. 4,414,023 dated Nov. 8, 1983 to Aggens et al. A specific ferritic stainless steel alloy useful in the devices hereof contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, and praseodymium, or a mixture of two or more rare earths, balance iron and steel making impurities.
In the following description, reference will also be made to fibrous ceramic mat or insulation. Reference may be had to U.S. Pat. No. 3,795,524 dated Mar. 5, 1974 to Bowman for formulations and manufacture of ceramic fibers and mats useful herein. One such material is currently available from 3-M under the registered trademark "INTERAM."
In the following description, reference will also be made to brazing foil. This foil is cast to about 0.001" to about 0.003" thick. It is desirably a nickel-chromium-boron-silicon brazing alloy analyzing 75% to 83% nickel with a liquidus temperature of 2100 F to 2300 F. Other nickel-containing brazing alloys 7% to 14% chromium, 3% to 4.5% iron, 3.5% to 4.5% silicon, 2% to 3% boron, balance nickel and having a liquidus temperature of above about 2100 F may also be used. Phosphorus in the alloy is to be avoided where platinum is used as the catalyst. Such alloys are available currently from Allied Metglas Products in Parsippany, N.J.
Many millions of automotive vehicles are equipped with catalytic converters, but virtually all are subject to start-up emissions of what, in at least one state, has been determined to be an unacceptable level. Anticipatory elevation of the catalyst temperature to an optimum operating level before start-up is expected to be mandated for many, if not all cars.
The metal monolith devices utilize, in the preferred embodiments, an elongated corrugated thin stainless steel strip, corrugated in such a manner as to be nonnesting when accordion folded, or when spirally wound about a central core. The corrugations may be, therefore, herringbone or chevron shaped, having a V-cross section with the apices rounded to reduce stress, or they may be straight through according to a variable pitch scheme such as described in U.S. Pat. No. 4,810,588 to Bullock and Whittenberger dated Mar. 7, 1989. Because of a tendency for the leading edges of corrugated thin metal strips at high gas space velocities on the order of 1,000,000 volume/volume/hour to roll over and induce destruction of the catalytic converter unit, it has now been found that a composite foil structure formed of corrugated and flat foil members will enable the core formed therefrom to withstand such velocities without failure.